1. Field of the Invention
The present invention relates to a direct current relay, and more particularly, to a direct current relay capable of reducing an electronic repulsive force generated between a fixed contact and a movable contact by a permanent magnet installed to extinguish an arc.
2. Background of the Invention
Generally, a direct current (DC) relay or a electromagnetic contactor is a type of electric circuit switching apparatus for performing a mechanical driving and transmitting a current signal using a principle of an electromagnet. The DC relay or the electromagnetic contactor is installed at various types of industrial equipment, machines, vehicles, etc.
FIG. 1 is a sectional view of a direct current relay in accordance with the conventional art, and FIG. 2 is an inner perspective view illustrating an upper part of FIG. 1.
The conventional direct current relay includes a lower frame 1, an upper frame 2, a pair of fixed contacts and a pair of movable contacts installed in the upper frame 2, and an electric actuator 5 installed in the lower frame 1 and configured to drive the movable contacts 4 so that a connected state between the fixed contacts 3 and the movable contacts 4 can be switched by an electric signal. Permanent magnets 6a, 6b are provided in the upper frame 2 so as to effectively control an arc generated when the contacts are separated from each other.
The pair of fixed contacts 3 are configured as a first fixed contact 3a and a second fixed contact 3b, and have polarities of (+), (−), respectively. The permanent magnets 6a, 6b installed in the upper frame 2 form a magnetic field (B), and are fixed by a permanent magnet holder (not shown). The magnetic field (B) generated from the permanent magnets 6a, 6b interacts with a current (+I, −I), thereby generating a force (+f, −f) to push out an arc generated when the contacts are separated from each other. This can reduce damage of a contact part.
However, the conventional direct current relay has the following problems.
Firstly, when the permanent magnets 6a, 6b for controlling an arc are provided at the direct current relay, a current (I) flows on the movable contacts 4 from a first movable contact 4a to a second movable contact 4b as shown in FIG. 2. Thus, a force (F) is applied to the movable contacts 4 in a downward direction by Fleming's law. The force (F) is applied in a direction to separate the movable contacts 4 from the fixed contacts 3. Such a force is called an ‘electrodynamic repulsion force’. In a normal current state, no problem occurs. However, when an over-current flows due to a fault current, the electrodynamic repulsion force is drastically increased, resulting in separation of the contact part. As a result, the fixed contact 3 and the movable contact 4 are separated from each other, and thus an inferior contact state may be caused.
FIG. 3 illustrates a magnetic flux density in a direct current relay in accordance with the conventional art, which shows a flow of a magnetic field (B) formed by the permanent magnets 6a, 6b. The magnetic field (B) flows in a direction toward the lower permanent magnet 6b from the upper permanent magnet 6a. It can be shown that a magnetic flux density within a range between the upper permanent magnet 6a and the lower permanent magnet 6b is almost constant.